The majority of individuals in need of drug abuse treatment remain untreated, and a sizeable proportion of drug abusers who enter treatment exit after a brief tenure. Thus, research that contributes toward an understanding of drug abusers' entry and engagement in drug abuse treatment, and their subsequent outcomes, is critical to efforts to improve treatment. Given the threat of HIV infection, improving treatment entry and engagement is particularly important with regard to opioid addicts, who are at especially high risk for HIV infection, and methadone maintenance treatment, which is widely used for the treatment of opioid dependence. A five-year services research study is proposed to comprehensively examine treatment entry and engagement, using a biopsychosocial theoretical perspective to guide the investigation. Informed by this perspective, the proposed study will examine three domains - namely, personal characteristics, treatment attributes, and environmental influences - that are hypothesized to affect opioid addicts' motivation to change and motivation for treatment, their entry into and subsequent engagement in methadone maintenance treatment, and, ultimately, their recovery. Within each of these domains, the effects of specific, potentially relevant variables, selected on the basis of previous research and theory, will be examined. Two groups of adult opioid addicts will be studied, including addicts who have not recently received nor are interested in seeking treatment (N = 200) and opioid addicts newly admitted to methadone maintenance treatment (N = 400), with both groups involved in the examination of treatment entry, and the admission group alone involved in the examination of treatment engagement. Comprehensive assessments will be conducted at baseline and 3 months (the 3-month assessment involving only the admission group to study their initial engagement in treatment), and 6 and 12 months thereafter. Two separate, but complementary, structural equation models are posited to predict (1) treatment entry and (2) treatment engagement and outcomes. These models, along with hypothesized components of these models, will be tested using structural equation modeling techniques, including the comparison of hypothesized models with plausible alternate models. While the primary study approach is quantitative, a strong qualitative research component will be included to provide for in-depth examination of motivation to change and motivation for treatment, and of other variables that are potentially important to understanding treatment entry and engagement.